To acquire better signal integrity, ZQ calibration is presented since DDR3 appeared for calibrating On Die Termination (ODT) and tracking pull-up/pull-down resistance regardless of Process-Voltage-Temperature (PVT) variations. Generally, the ZQ calibration is performed by the following steps: comparing a reference signal and a target signal by a comparator; generating a calibration code by a ZQ calibration controller depending on the comparison output; applying the calibration code to a resistance calibration unit (pull-up/pull-down resistance calibration unit) by the ZQ calibration controller to adjust the target signal via the resistance calibration unit; and replacing the target voltage with the adjusted target voltage and repeating the above steps until the target voltage is equivalent or almost equivalent to the reference voltage. Depending on system environment with various noises and interferences, the comparison output of the comparator may be incorrect or misjudged. Once the comparison output is incorrect or misjudged, the calibration code generated by the ZQ calibration controller may depart much from the final calibration code which drives the target voltage to get on for the reference voltage, especially as the ZQ calibration controller use a dichotomy search method to generate the calibration code. In view of this, it is important in the art to avoid that a comparator provides an incorrect comparison out for a ZQ calibration controller in ZQ calibration of a memory device.